It is now recognised that some of the nitrogenous chemicals, many of them accelerators, used in processing rubber can generate nitrosamines, many of which are powerful carcinogens. Typical such chemicals include morpholino(di)thiobenzthiazole, dithiomorpholine, tetramethylthiuram disulphide and zinc dimethyldithiocarbamate. They give rise to 4-nitrosomorpholine and dimethylnitrosamine, both of which are subject to regulation in some countries.
Various approaches have been adopted, with a view to reducing the occurrence of such hazardous nitrosamines. For example, in some cases rubber can be vulcanised without using nitrogenous chemicals, e.g. by means of diisopropylxanthogen disulphide activated by zinc isopropyl xanthate; see EP-A-0184301. Another approach is to use chemicals based on amines which do not form carcinogenic nitrosamines, e.g. zinc dibenzyl dithiocarbamate, tetrabenzylthiuram di- and tetra-sulphides (see EP-A-0284649, EP-A-0284650, DE-A-4027114 and EP-A-0406603) or derivatives of amines lacking a labile hydrogen on the .alpha.-carbon atom (Spiegelhalder). A further approach is to include nitrosamine decomposers in the processing stages. Such decomposers are typically primary amines, ammonia or ascorbic acid; see EP-A-0455009 and EP-A-0463465.
These approaches have certain disadvantages. For example, in the case of xanthogen sulphides, a nitrogenous co-agent is often essential to achieve satisfactory vulcanisate properties. Amines which form non-carcinogenic nitrosamines do so because of their particular structures and these same structures are disadvantageous as regards accelerator action in rubber. Nitrosamine decomposers can easily become separated from the nitroso-species they are intended to destroy, and thus lose their effect.